[FZ Editor] add opacity to editor's background

[enricogior]

The FZ Editor has a transparent window that covers the entire screen, adding opacity improves the editor experience in my opinion.

I've put on the before and after in same image, in the picture the difference might not be so significant, but it makes the editor much cleaner.
This is the preview mode:

This is while editing the layout:

I didn't spend time playing with different combination of color, so this is just a POC, we can consider other color option for the background and/or the zones.
We should also start thinking about the colors for the light and dark themes.

[enricogior]

@niels9001
what do you think?

[niels9001]

Oooh, I like it!

Would be nice to align the semi-transparent backdrop with the one that is in Shortcut Guide and a similar issue tracked for PT Run: #2448

Things to consider:

• Dark / light theme support (black backdrop / white backdrop)
• Aligned settings (shortcut guide allows customization for this - maybe we should make a generic one for FZ editor, SG and PTR). I think allows transparency and color setting.

@enricogior does that makes sense? Because maybe we should create one issue to create a 'specification' for this for current modules and future ones?

[enricogior]

Yes it makes sense.
We can then link to the individual module issues like this one (we don't have to fix all in the same PR, we can define the specification and then implement it in each module).